Silver alloys having high sulphuration resistance

ABSTRACT

Tarnish-resistant silver alloys which retain the malleability and silver-white appearance of the pure metal containing tin, indium and zinc in specified concentration ranges. These elements are synergistic in their effects. Small quantities of Ti, Zn, Be, Cr, Si, Al, Ge and Sb when used in addition to Sn, In and Zn further increase resistance to tarnishing by sulphur-containing materials.

United States Patent 1 Harigaya et a1.

[451 May21, 1974 1 SILVER ALLOYS HAVING HIGH SULPHURATION RESISTANCE [75] Inventors: Hiroshi Harigaya; Kazutomo Kasai,

both of Suwa; Michio Asahina, Okaya, all of Japan [73] Assignee: K bushiki Kaisha Suwa Seikosha, Tokyo, J @535? [22] Filed: July 13, 1972 [21] Appl. No.5 271,326

Related us. Application Data [63] Continuation of Ser. No. 5,917, Jan. 26, 1970,

abandoned.

[30] Foreign Application Priority Data Feb. 5, 1969 [52] C1. 75/173 R, 75/134 T, 75/173C [51] Int. Cl. C22c 5/00 [58] Field of Search .75/173 R, 173 C, 134 T [56] I References Cited UNITED STATES PATENTS 1,847,941 3/1932 Gray et al. 75/173 R 'Japan 44-8289 Primary Examiner-L. DewayneRutledge Assistant Examiner-E. L. Weise Attorney, Agent, or Fi'rm-Blum, Moscovitz, Friedman & Kaplan 57 ABSTRACT Tamish-resistant silver alloys which retain the malleability and silver-white appearance of the pure metal containing tin, indium and zinc in specified concentration ranges. These elements are synergistic in their effects. Small quantities of Ti, Zn, Be, Cr, Si, Al, Ge and Sb when used in addition to Sn, 1n and Zn further increase resistance' to tarnishing by sulphur-containing materials. v

- 7 Claims, No Drawings ,1 SILVER ALLOYS HAVING HIGH SULPHURATION RESISTANCE This is ,a continuation of application Ser. No. 5,917, filed Jan. 26, 1970 now abandoned.

The present invention relates to silver alloys for omamerits and industrial art products, comprising Sn of 4-10 percent, In of 0.5-l2 percent,'Zn of 0.1-5 percent and substantially Ag of the remainder by weight wherein noble bright white color of silver alloys is maintained and resistance against discoloration into black, due to sulphuration, is greatly improved by the synergism (effect) which is caused from the addition of such elements as Sn, Inand Zn into Ag at a time.

As is well known, Ag having its own bright white color and soft color tone is valued as a precious metal; for example, it is widely applied to some ornaments such as rings, silver tablewares, watch cases or to dentistry.

Silver, however, has defects that if it reacts with a small amount of sulfide in the air or sweat, silver sulfide is produced instantly and thus discoloration from silver white to black is broughtabout: It is also a well-known fact that it is restrictedin its usage.

Silver alloys that do not change to black by silver sulfide have been widely sought. However, satisfactory silver alloys have not been obtained; But as the alloy making up these defects, silveralloys, including Pd and'Au etc., arecommonly" known, for-example, the alloys being employed by dentists.

Thesealloys are'improved in their sulphuration resistance, whilethe bright white color of Ag own, as mentioned-above, is tinged with grey and is too expensive. Most articles, such as watch cases, etc., for practical use or" industrial products, are commonly plated with Rh, etc., to protect their surfaces, so that the natural color ofAgcannot be seen.

The purpose of the present invention is to eliminate the above defects and to provide silver alloys which can retain the'natural-silver white color of Ag and prevent the'discoloration to black due to sulphuration during their use, and also the-costs must be either the same as that of Ag or lower than Ag.

Many'experiments have been carried out to obtain such alloys. As aresult of such experiments, it was proved that proper addition of Sn to Ag increases the sulphuration resistance and the mechanical strength, and the effect of In in improving sulphuration resistance to a great degree together with Sn,-and further addition of Zn with Sn and In makes it possible to realize the alloys accordingjto the present ivention.

The effect of each additional element of the alloys according to the invention and the reason for restricting the amount of the element will be described hereinafter.

Sn becomes oxide preferably in; the oxygen gas and becomes stable, thus protecting the surface of the alloy and prevents silver sulfide from growing. The additionalamount of 4 percent (Sn) begins toshow the desirable effect on the sulphuration resistance. As the amount is increased, the effect and the mechanical strength also increase. However, if the additional amount exceedslOperce'nt, the color'of silver proper turns to gray. In addition, the'silver alloys including Sn become brittle and the workings, such as drawing and rolling would become enveloped in difficulties. Thus it could be found thatSn of 4-10 percent is sufficient.

In is very soluble in Ag, the elements however having poor corrosion resistance. If In of more than 10 percent cent In shows a favorable effect on the sulphuration resistance, and as the additional amount of In is increased the effect is also increased. However, if the additional amount of In exceeds 12 percent, the alloys become tinged with yellow and their malleability is decreased. Therefore, the additional amount of In must not exceed 12 percent.

' Zn has deoxidizing action on the alloys when they are melting. GenerallyZn shows a favorable corrosion resistance in the air. In the solution, including salts, it also prevents the alloys from being corroded, that is, corrosion resistance of the alloys is highly increased. Even with a comparatively small amount of Zn to be added, it is effectiveas deoxidizing agents. In order to increase the. corrosion resistance and the sulphuration resistance, an amount of more than 0.1 percent is needed.

If the additional amount of Zn exceeds 5 percent, the alloys become tinged with grayand become brittle,

' which has a deleterious effect on their malleability and ductility. Therefore it is fixed in the amount of 0.1-5

percent. I

According to the invention, as can be understood from the additional elements and their amount above stated, good properties of Ag proper can be maintained for a long time and the discoloration to black, due to sulphuration, which is the largest defect in Ag, can be prevented by adding some elements, mixed together,

the prices of which are approximately the same as Ag or lower than Ag. Furthermore, the addition. of Sn and In permits the mechanical strength of the alloys to increase greatly. Thus, the alloys, according to the present invention, can be applied not only'to ornaments or technical products but also to various industrial uses.

The present invention will be disclosed in detail with the explanation of the following embodiments.

EXAMPLE 1 a The alloy comprising Sn of 8.2 percent, In of 2.1 percent, Zn of l.2 percent and'Ag of theremainder by weight, this alloy being called thealloy No. l" according to the invention hereinafter, was melted and founded. After being subjected to repeated heat treatment and rolling process, it was formed intothe plates having a thickness of 2 mm. Further a test piece 30 mm by 30 mm was cut off from the plate. After the surface of the test piece was mirror polished by emery papers and a buff polishing machine, it was'washed out to remove oils and fats and was kept in a sealed container, in which 6N (normal) potassium sulfide of cc and 2N. potassium sulfide of 200 cc were poured and hydrosulfide gas was generated. After keeping the test As a result, no discoloration was found and the initial tional silver alloys. But it is desirable to add Zn before surface luster and tone could be maintained in the alloy founding into ingot in order to fully display its deoxidiz- No. 1 according to the present invention. ing effect. It is especially desirable to reduce the Moreover three kinds of alloys the one comprising amount of oxygen absorbed under high temperature. Sn of 8.9 percent and Ag of the remainder (Sn is added The conventionally used phosphoric fluxes etc. as desingly), the one comprising In of 9.7 percent and Ag of oxidizing agents have an extremely deleterious effect the remainder, and the one comprising Sn of 8.7 peron the sulphuration resistance in the alloys according cent, In of 4.7 percent and Ag of the remainder (a mixto the present invention. Thus they must not be used. ture of Sn and In is added) were respectively melted Similarly S must also be prevented from being mixed in and founded. They were then formed into the same test by accident at melting. The founding is carried out by pieces as beforementioned- Thereafter, they were a melt mold etc. After face scraping, the alloys are sub- Placed in hydrosulfide g and examined- The result jected to the hot or cold rolling. The workability is very was as follows: The alloy including Sn added singly and hi h, Th heat treatment during processing is between the alloy including In added singly were discolored re- 500 a d 650 C, As for the cooling method, a quick spectively Within 5 te While the all y n ng cooling is more effective than a slow one from the point S and mixed g h began to discolor to light of view of workability or sulphuration resistance.

brown in its Surface a er S t) h g The alloys shown in the example were produced in From h above experiment. It was found that the Slllthe same manner as mentioned above. The final reducphuration resistance of Ag would be more highly intion rate was 30 percent (cold rolling) and the alloys creased by the synergism Caused y e mixed addition were formed into the plates having a thickness of 2 mm, In a n at {tn H W after which test pieces were cut ofi from them respec- The alloy including Sn added singly shows white gray tively for examination of their properties. The result is tone, and the alloy including ln added singly shows silshown in Table 2.

MM- i T551 82 Alloy No. sulphuration ra nge Hv after according to hydro sulfide artificial cold color tone the invention gas sweat rolling Alloy no. 2 O O l 18 silver white do. 3 O 147 semi yellow white do. 4 165 silver white do. 5 155 do. do. 6 g I48 1 do. do. 7 148 do. do. 3 0 I52 do. do. 9 O 153 do. do. 10 g 0 165 do. do. ll Q 142 do. Purc silver .r .r 77 pure silver Sterling silver x .r 98 semi yellow white Au-Pd-alloy O C 142 gray ver color tinged with light yellow. On the other hand, I In the sulphuration resistance test, the alloys were the alloy No. 1, according to the invention can mainkept in the hydro sulfide gas in the same manner as tain soft silver white tone of Ag proper. shown in the Example 1. The surface condition of the test pieces was then examined after 6 hours. The result EXAMPLE 2 is shown by the following signs. The sign ofOshows no discoloration. The sign of 0 shows a little discolor- Table 1 shows the chemical compositions of the alation," the sign of x shows discoloration to black," loys according to the present invention and the convenwhile the pure silver and the Sterling Silver discolored tional silver alloys. to black within 5 minutes.

" Table 1" i Alloy No. according Sn ln Zn Ag to the invention Pure silver 99.9% up 92.5% Sterling silver (Cu 7.5%)

Au-Pd-alloy (Pd 25% Au 5%) The melting process of the alloy according to the Furthermore, the sulphuration resistance test was present invention can also be conducted by using the carried out in the artificial sweat as follows: carbon crucible in the same manner as that of conven- The half portion of the test piece was immersed in thc COMPOSITION OF ARTIFICIAL SWEAT NaCl 0.648 0.987% N'agS 0.006 0.025% (NH,),C 0.086 0.173% NH 0H 0.0l0 0.018% Saccharose 0.006 0.022% Lactic acid 0.034 0.107% H,0 remainder rare (or in a vacuum) to make a stronger oxidized film It can be understood from the above examples that the alloys according to the present invention have a high sulphuration resistance. Asfor the hardness, the. alloys showed about 160 Hv by a 30 percent cold rolling. It was found thatthe alloys according to the invention have approximately-the same mechanicalpropenties as brass. 1

After various experiments, the'inventors also found that the mechanical strength and the sulphuration resistance could be increased still higher '-by minimizing crystal grains and making the quality uniform by adding a very small amount of sub-additional elements such as Ti, Zr, Be, Cr, Si, Al, Ge and Sb. These elements-are alloyed and act to make the Ag atom stable and at the same time act to protect the surfaces of the alloys by oxidizingthem'selves preferentially, all of these elements making the very stable oxides. Therefore, the sulphuration resistance can be further increased by. making the oxidizedfilm intentionally to protect the surface of the silver alloy.

ln otherv words, the alloys can be heated up to 100. 300 C in the atmosphere where oxygen is moderately than that in a room temperature. In this case, however, it is important to select the conditions such as atmosphere, temperature, etc. so the luster of the alloy surface by the oxidized film is not lost. If the additional amount of sub-additional element is less than 0.001 percent the favorable effect cannot be obtained. If Ti, Zr, Be, Cr and Si of more than 1 percent is added respectively, the alloy becomes extremely brittle and hardened, and further since insoluble elements are yielded, a fine luster; of the alloy surface cannot be obtained at the mirror-polishing. The additional amount .is, therefore, suitable in the range of 0.00l-l percent.

Further, respective additions of Al, Ge and Sb of 5 percent at the maximum does not diminish the workability and surely have favorable effects on the sulphuration resistance and the mechanical property. The additional amount is fixed in the range of 0.001- 5 percent.

These sub-additional elements have the same action. If more than two kinds of these elements are added as a mixture they do not diminish the effect of their own actions. Therefore, these sub-additional elements can be added not only singly but mixed together (more than two kinds). The latter is more effective than the former.

The effects of the sub-additional elements will be explained by-way of the following embodiment.

EXAMPLE 3 Table 3 shows the chemical composition of the alloys shown in the example. Table 4 shows the properties of the alloys above. These are the result of the examination in the same manner as shown in example 2. However, the keeping periods at the sulphuration resistance tests in hydrosulfide gas and in artificialsweat are 10 hours and 60hours respectively.

As is apparent from Table 4, the sulphuration resistance and the mechanical property are greatly increased by adding the sub-additional elements. The

' alloy No. 15 according to the invention, for example,

shows 50-52 kglrnm in its tensile strength. This can compare with a brass inhardness. As Ti, Zr, Cr, Be and Si among these elements have high melting points, they are formed into mother alloys with Ag at melting and then added in the form of mother alloys. Further, they are easy to oxydize under a high temperature. Thus it is desirable to use some suitable fluxes or otherwise to melt in an inactive gas in order to prevent the loss of the melt due to oxydization at melting.

In the alloys, according to the present invention, the workings 'such as melting, founding and rolling, etc., are easily carried out, but if a relatively large amount of Sn and In are added, or the oxides are mixed in when a melting process is not carried out successfully, the workability (malleability and ductility) may be slightly decreased.

' As to the above, various experiments and examinations were repeated. The inventor found that it is effec- -In are added in great amounts, while Zn in small amount, the effect mentioned above is especially large.

f T ahle 3 T Alloy No; according Sub-additional to the invention Sn In Zn Ag element Alloy No. l2 8.3 2.1 L0 remainder non Ge'lS Table 4 Alloy No. sulphuration range Hv. after according to hydro sulfide artificial 30% cold color tone the invention gas sweat rolling Alloy No 12 O O 139 silver white do. 13 8 155 do. do. -l4 163 do. do. 15 8 153 do. do. 16 165 do. do. 17 O O 151 do. do. 18 I 163 do. do. 19 0 Q 168 do. do. 20 9 l5.9 .dodo. 21 z z 170 do. do. 22 1 162 do. do. 23 O 0 175 do.

With the additional amount of more than 0.001 percent, thev effect is shown. However, if it exceeds the solution limit, the sulphuration resistance of Ag is at times decreased. Thus it is fixed in the range of 0001- percent.

The effects in improving workability and foundability by addition of Cu will be disclosed with the explanation of the following embodiment.

The inventors confirm the superiority of the alloys according to the invention not only by the experiments in the laboratory, but also by actually producing and using the ornaments, tablewares or watch cases, etc. Furthermore, since the silver alloy according to the invention has a mechanical property as high as brass, high plasticity and higher free cutting property than that of a free cutting brass, it can be widely applied to practical uses and industrial uses.

EXAMPLE 4 Table 5 shows the chemical composition of the alloys one with Cu added, the other without Cu.

Table 5 Alloy No. according to the invention Sn In Zn Ag Cu Alloy No. 24 6.7% 3.4% 1.8% remainder 25 7.2 1.5 1.4 do. 0.4% 26 6.7 3.5 1.8 do. 0 27 7.4 1.6 1.5 do. 0

What is claimed is:

l. A tarnish-resistant, easily workable silver-base alloy having the characteristic appearance of pure silver consisting essentially of 4-10% of Sn, 05-12% of In, and 0.1-5% of Zn by weight, the remainder being silver.

2. A tarnish-resistant, easily workable silver-base alloy having the characteristic appearance of pure silver consisting essentially of 4-10% of Sn, 05-12% of In, 0.1-5% of Zn by weight and at least one member of the group consisting of Ti, Zr, Be and Cr wherein the total quantity of said member or members is in the range of .001 to 1 percent by weight, the remainder of said silver-base alloy being silver.

3. A tarnish-resistant, easily workable silver-base alloy having the characteristic appearance of pure silver consisting essentially of 4-10% of Sn, 05-12% of in, 0.1-5% of Zn by weight, and at least one member of the group consisting of Sb, Al and Ge, wherein the total quantity of said member or members is in the Table 6 Alloy No. accord- Yield Rate Sulphuration ing to the invenlngot By Face workability resistance tion Surface Scraping Alloy No. 24 good 79% good 0 do. 25 good 77 extremely good Q do. 26 average 67 average 0 do. 27 average 6 1 good Q As described above, the alloys according to the present invention comprise Ag, Sn, 1n and Zn as main elements. Ti, Be etc. as sub-additional elements for improving the sulphuration resistance and the mechanical property, and Cu for improving foundability and the workability. The addition of Cu and that of subadditional elements show entirely different effects respectively. Thus even if these elements are added as a mixture, their own effects are not spoiled.

As can be understood from the above description, the silver alloys according to the invention have extremely high sulphuration resistance compared with that of the conventionalsilver alloys.

range of 0.001-5 percent by weight, the remainder said silver-base alloy being silver. v I

4. A tarnish-resistant, easily workable silver-base alloy having the characteristic appearance of pure silver consisting essentially of 4-10% of Sn, 0.5-l2% of In, 0.1-5% of Zn by weight, at least one member of the group consisting of Sb, Al and Ge, wherein the total quantity of said member or members is in the range of 0.001-5 percent by weight, and at least one member of the group consisting of Ti, Zr, Be and Cr wherein the total quantity of said member or members is in the range of .001-1 percent by weight, the remainder of said silver-base alloy being silver.

5. a tarnish-resistant, easily workable silver-base alloy having the characteristic appearance of pure silver consisting essentially of 4-10% of Sn, 05-12% of In, 0.l5% of Zn by weight, containing 0.001-l% of Cu by weight and atleast one member of the group consisting of Ti, Zr, Be and Cr wherein the total quantity of said member or members is in the range of .001-1 percent by weight, the remainder of said silverbase alloy being silver. j

6. A tarnish-resistant, easily workable silver-base alloy having the characteristic appearance of pure silver consisting essentially of 4-10% of Sn, 0.542% of In, 0. l5% of Zn-by weight, containing 0.001I0% of Cu by weight, and at least one member of the group 1 consisting of Sb, Al and Ge, wherein the total quantity of said member or members is in the range of 0.001-5 percent by weight, the remainder being silver.

7. A tarnish-resistant, easily workable silver-base alloy having the characteristic appearance of pure silver consisting essentially of 4-10% of Sn, 0.5-l2% of .001 to 1 percent by weight, the remainder being silver. 

2. A tarnish-resistant, easily workable silver-base alloy having the characteristic appearance of pure silver consisting essentially of 4-10% of Sn, 0.5-12% of In, 0.1-5% of Zn by weight and at least one member of the group consisting of Ti, Zr, Be and Cr wherein the total quantity of said member or members is in the range of .001 to 1 percent by weight, the remainder of said silver-base alloy being silver.
 3. A tarnish-resistant, easily workable silver-base alloy having the characteristic appearance of pure silvEr consisting essentially of 4-10% of Sn, 0.5-12% of In, 0.1-5% of Zn by weight, and at least one member of the group consisting of Sb, Al and Ge, wherein the total quantity of said member or members is in the range of 0.001-5 percent by weight, the remainder of said silver-base alloy being silver.
 4. A tarnish-resistant, easily workable silver-base alloy having the characteristic appearance of pure silver consisting essentially of 4-10% of Sn, 0.5-12% of In, 0.1-5% of Zn by weight, at least one member of the group consisting of Sb, Al and Ge, wherein the total quantity of said member or members is in the range of 0.001-5 percent by weight, and at least one member of the group consisting of Ti, Zr, Be and Cr wherein the total quantity of said member or members is in the range of .001-1 percent by weight, the remainder of said silver-base alloy being silver.
 5. a tarnish-resistant, easily workable silver-base alloy having the characteristic appearance of pure silver consisting essentially of 4-10% of Sn, 0.5-12% of In, 0.1-5% of Zn by weight, containing 0.001-10% of Cu by weight and at least one member of the group consisting of Ti, Zr, Be and Cr wherein the total quantity of said member or members is in the range of .001-1 percent by weight, the remainder of said silver-base alloy being silver.
 6. A tarnish-resistant, easily workable silver-base alloy having the characteristic appearance of pure silver consisting essentially of 4-10% of Sn, 0.5-12% of In, 0.1-5% of Zn by weight, containing 0.001-10% of Cu by weight, and at least one member of the group consisting of Sb, Al and Ge, wherein the total quantity of said member or members is in the range of 0.001-5 percent by weight, the remainder being silver.
 7. A tarnish-resistant, easily workable silver-base alloy having the characteristic appearance of pure silver consisting essentially of 4-10% of Sn, 0.5-12% of In, 0.1-5% of Zn by weight, containing 0.001-10% of Cu by weight and at least one member of the group consisting of Sb, Al and Ge wherein the total quantity of said member or members is in the range of 0.001-5 percent by weight, and at least one member of the group consisting of Ti, Zr, Be and Cr, wherein the total quantity of said member or members is in the range of .001 to 1 percent by weight, the remainder being silver. 